Measurements and ab initio Molecular Dynamics Simulations of the High Temperature Ferroelectric Transition in Hexagonal RMnO3

Measurements of the structure of hexagonal RMnO3 (R=rare earths (Ho) and Y) for temperatures significantly above the ferroelectric transition temperature (TFE) were conducted to determine the nature of the transition. The local and long range structural measurements were complemented by ab initio molecular dynamics simulations. With respect to the Mn sites in YMnO3 and HoMnO3, we find no large atomic (bond distances or thermal factors), electronic structure changes or rehybridization on crossing TFE from local structural methods. The local symmetry about the Mn sites is preserved. With respect to the local structure about the Ho sites, a reduction of the average Ho-O bond with increased temperature is found. Ab initio molecular dynamics calculations on HoMnO3 reveal the detailed motions of all ions. Above ~900 K there are large displacements of the Ho, O3 and O4 ions along the z-axis which reduce the buckling of the MnO3/O4 planes. The changes result in O3/O4 ions moving to towards central points between pairs of Ho ions on the z-axis. These structural changes make the coordination of Ho sites more symmetric thus extinguishing the electric polarization. At significantly higher temperatures, rotation of the MnO5 polyhedra occurs without a significant change in electric polarization. The born effective charge tensor is found to be highly anisotropic at the O sites but does not change appreciably at high temperatures

DEVICE APPLICATIONS OF GARNET THIN FILMS

Les 15 dernières années ont été très fructueuses pour le développement de dispositifs réalisés avec des films minces de grenats. Les principaux domaines d'application sont les mémoires à bulles magnétiques, les mémoires optiques, les dispositifs d'affichage, les dispositifs hyperfréquences à ondes magnétostatiques, mais également d'autres domaines tels que les composants optiques intégrés pour les télécommunications sur fibre optique. L'objectif de cette revue est d'examiner les développements récents des composants en relation avec les propriétés souhaitées pour les films de grenat : anisotropie magnétique, aimantation, rotation Faraday, largeur de raie de résonance ferromagnétique, etc... Les composants seront décrits et les problèmes techniques restant à résoudre seront discutés.The past 15 years have been very fruitful in the field of development of devices making use of garnet thin films. The major device areas are the magnetic bubble memories, the magneto-optical displays and printers, the optical memories, the magnetostatic surface wave filters, but, also, other devices such as integrated optics components are of current interest. The purpose of this review is to examine the recent developments of components in relation to the suitable properties of the garnet films : magnetic anisotropy, magnetization, Faraday rotation, FMR linewidth, etc... The components will be described, as well as technical problems still to be solved